By gas diffusion electrode is understood a porous catalytic electrode in which a gas reactant and an aqueous electrolyte are brought into contact to react electrochemically. Gas diffusion electrodes are used extensively in electrochemical power sources such as metal-air and metal-hydrogen batteries or fuel cells. In all of these, the gas diffusion electrode is on one side in contact with an electrolyte and on the other side in contact with a gas. In the metal-air battery, the gas is air.
Attempts have already been made to provide gas diffusion electrodes which operate alternately for gas consumption and gas evolution. These electrodes, which combine cathodic and anodic functions, are known as bifunctional gas diffusion electrodes. A typical application of these bifunctional electrodes is in secondary metal-gas batteries such as iron-air, zinc-air and nickel-hydrogen batteries which consume gas during discharge and evolve gas during electrical recharging.
German Patent application No. DT 19 21 157 describes a rechargeable metal-air battery with a bifunctional gas diffusion electrode comprising two electrode layers bonded to each other: a hydrophilic electrolyte-side layer made of porous nickel and a hydrophobic gas-side layer made of carbon, a PTFE binder and preferably impregnated with an oxygen-reducing catalyst. The hydrophilic porous nickel layer has a dual function, as a current collector and as an active layer for oxygen evolution during charge. The gas-side hydrophobic layer is an active area of the electrode during the gas consumption phase or discharge cycle of the battery. Thus, the two active zones of the electrode during charge and discharge were theoretically kept apart, but some unwanted oxygen evolution took place in the hydrophobic gas-side layer during charge and, despite attempts to prevent this by an increase of pressure on the gas-side of the electrode, was the cause of corrosion, deactivation of the catalyst and a reduced lifetime.
Efforts to improve these bifunctional electrodes, in particular to obtain an acceptable lifetime, have not been successful to date with the result that metal-gas batteries still have a relatively limited use.